1. Field of the Invention
This invention relates to polyester films having improved slip properties and durability and suitable for use in such applications as require a smooth surface. More particularly, it relates to a polyester film suitable for use in magnetic tapes which provides improvements in many respects including abrasion resistance and adhesion of magnetic layers thereto in tape making process as well as electromagnetic conversion characteristics, abrasion resistance, traveling property, etc. of magnetic tape products.
2. Description of the Prior Art
Polyester films, particularly polyethylene terephthalate films have been widely used as substrates in many applications such as magnetic tapes, capacitors, packages, printing plates, electric insulators, photographic films and the like because of their superior mechanical and electric properties and good chemical resistance and dimensional stability.
In particular, the use of polyester films as base films of magnetic tapes has outstandingly been expanded and they have been used in magnetic tapes for various purposes including audio, video and computer tapes. As a result, the requirements for the characteristics and properties of these tapes have become increasingly strict in recent years.
Among others, video recorders, particularly home video recorders, have spread at an outstanding rate and some home video recorders recently appearing on the market possess various functions such as fast moving, standing and slow moving of the picture and even intermittent tape feed.
Those tapes intended for use as video tapes must have the ability to always maintain a sharp and non-distorted picture and sound even when various operations of video recorders as mentioned above are repeated several hundred times in the surroundings of varying temperatures and humidities. In this respect, magnetic layers or coatings have, of course, undergone various improvements. In addition, the requirements for the characteristics and properties of polyester films used as base films for video tapes are much more strict than those of polyester films for audio tapes.
Among important properties required of base films for video tapes are abrasion resistance and adhesive property that are necessary during tape making process, so-called electromagnetic conversion characteristics which affects recordability and reproducibility of picture and sound of tape products, and abrasion resistance and traveling property which may cause some troubles during use of tapes.
Abrasion of such films may occur when a film or tape is exposed to considerable friction during film producing process, during tape making process prior to the step of coating the film with a magnetic layer, and during recording or playback of information. More specifically, when a polyester film is passed through a nip of rotating rolles or around a stop roll or free roll, etc., the film is worn away with generating abrasion powder (dust) out of the surface thereof. Such dust may contaminate the recording and playback system of the recorder, thereby causing various troubles.
Traveling property means the uniformity of traveling speed of a tape. Poor traveling property causes unevenness of traveling speed, which in turn leads to decrease in output of sound and picture quality of the tape and even to unstable traveling or failure of travel after repeated passes of the tape. Traveling property of a film has relation to the coefficient of friction of the film. As a matter of course, a film having poor slip property shows a poor traveling property. At present, there is an increasing need for long period tapes which is accomplished with a limited cassette size by tape feed at a low speed. Therefore, an improvement in traveling property, particularly at low traveling speeds is still desired.
The electromagnetic conversion characteristics of magnetic tapes, particularly video tapes have close relation to reproducibility of sound and picture. A magnetic material having a high residual magnetism and a high coercive force is employed in these tapes in order to attain a high output over a wide range of frequency ranging from several ten hertz (Hz) to several megahertz (MHz). In order to minimize the noise level (S/N ratio) each of color and brightness signals, reduction in particle size of the magnetic material and uniformly distributed coating thereof are necessary together with a certain finishing technique and the use of a base film of high quality. In order to achieve high density recording as a countermeasure for compaction of home video and other recorders, and S/N ratio should be improved. For this purpose, a magnetic layer must have a smooth surface so as to attain sufficiently close contact between the tape and video heads to minimize the so-called spacing loss. This also requires the underlying base film to have an improved thickness precision and surface smoothness. However, a film having a smooth surface is inferior in the foregoing abrasion resistance and traveling property.
Thus, in order to improve a tape in each respect of electromagnetic conversion characteristics, abrasion resistance and traveling property, the surface roughness of the base film needs to be adjusted or modified in opposite two directions which are incompatible with each other. In other words, the improvement in electromagnetic conversion characteristics can be accomplished generally by controlling the film surface in the direction of smoothing, while the improvement in abrasion resisting and traveling properties can be accomplished generally by controlling the same in the direction of roughening. It is very difficult for a film to have all the above-mentioned properties that are inconsistent with one another. For this reason, in base films for magnetic tapes currently available on the market, particularly in those films for video tapes that have more strict requirements for the abrasion resisting and traveling properties, primary importance is placed on roughening of film surface at a certain sacrifice of electromagnetic conversion characteristics.
As an attempt to provide a base film with each of the above-mentioned properties, it has heretofore been proposed to employ a double-layer laminate film having a smooth surface on the side to be coated with a magnetic layer and a rough surface on the opposite side. Also it has been proposed to subject the surface of a base film to chemical treatment only on one side. These techniques, however, cannot provide satisfactory solutions since even with these techniques it is not impossible to avoid contact of the smooth surface with a roll during film processing. In addition such techniques are cost-consuming so that they have not yet received practical application. The so-called back coating method in which a base film is coated with such material as carbon on the side facing away the magnetic layer also involves various problems in that it adds to the complexity and cost of the film processing and that it goes against the current tendency to reduce tape thickness in order to accomplish long-period tapes.